This invention relates generally to video signal processing and is particularly directed to the detection and processing of vertical sync pulses in a composite video signal.
A typical video signal incorporates three primary components: a picture signal, a composite synchronization signal, and a blanking signal. The picture signal includes video information such as luminance and chrominance signals. The composite synchronization signal includes pulse-like signals occurring at predetermined horizontal and vertical scan rates which are interspersed between the scan interval picture component in the period generally referred to as the retrace interval. These sync pulses coincide in time with the blanking signals which are used to blank the video during retrace.
The video signal is formed by scanning an image from top to bottom with a plurality of sequential horizontal scan lines. The scan is blanked during horizontal retrace intervals between adjacent scan lines as well as during vertical retrace intervals between successive fields. A plurality of horizontal sync pulses define horizontal timing, while a vertical sync pulse defines vertical timing between successive fields. The vertical sync pulse is defined by six broad pulses in accordance with NTSC standards. In addition, first and second groups of equalizing pulses occur just before and just after the vertical sync pulse during the vertical retrace interval. The equalizing pulses, which occur at twice the line frequency, serve to reduce the effect of line-frequency pulses on the interlaced line scansion of the image and to coordinate vertical timing for proper interlace.
The typical television signal processor separates the vertical sync pulses from the horizontal sync pulses by passing the composite sync waveform through an integrator circuit which smoothes the narrow horizontal sync pulses and provides a rapidly rising voltage for the vertical broad pulses. A slicer is typically set to about one-half the pulse height for providing a sharp output pulse to identify the vertical sync pulse. Various factors in the video signal transmission environment increase the difficulty of synchronization pulse detection in such a pulse integration arrangement. These factors can produce excessive sync pulse amplitude variations, loss of low or high frequency components, imposition of white noise on the sync signal or the super-position of impulse noise on the incoming sync pulse, all of which increase the possibility of erroneous sync. For example, a noisy environment tends to break up, or crack, the vertical sync pulses making them more difficult to detect. In addition, the integration of the various broad pulses places an inherent limitation on the accuracy of detection of the width of the vertical sync pulse.
The present invention is intended to overcome the aforementioned limitations of the prior art by comparing the width of the vertical sync pulse with a precise three horizontal line reference. If the vertical sync pulse width is correct, then a vertical reference signal is provided to a vertical pulse generator for generating field rate pulses. The present invention edge detects each of the broad pulses in the vertical sync pulse rather than accumulating a charge corresponding to their sequential occurrence in providing for the accurate and reliable detection of vertical sync. Although the present invention is particularly adapted for use in a cable television (CATV) encoder, it has application in any system wherein composite video signals are processed.